Spectral DMC-30

Dear brothers, can anybody tell me the function of the two black-color knobs, calibrated 1-10, found at the rear when the top lid is opened. They used to be sort of
'gain' control with the DMC-20, but it is not the case with the 30s. I got this pre-amp. as used equipment and hence do not have the owner's manual. Can somebody looked up for me? Thanks